Maintenance of the airfoil shape of a sail is critical to its performance. Conventional woven, knit, or scrim sailcloth, whether of natural fibers or the latest polyester, nylon, aramid, PEN, PBO, ultra high molecular weight polyethylene (UHMWP), or carbon fiber, is prone to stretch, creep elongation, and airfoil shape deformation because of non-linear forces on the sailcloth under load, particularly when sailing upwind. When sailcloth is stretched and deformed, the airfoil shape of the sail is deformed, and the lifting capability of the airfoil is degraded. Since the mid 1970s, a number of load force advances have been made in the design of sails, in the use of different materials, and in methods of constructing sails, all intended to limit stretch, creep, elongation, and deformation of sailcloth and sails. These advances range from ever more tightly woven fabrics, development of more resilient fibers, alternative designs for sail panels, computer aided analysis of load forces, and computer aided design and manufacture of integrally interconnected sailcloth panels for individual sails, computer aided design of the direction of the thread or fiber layout of each panel of individual sails, and computer aided, three dimensional “molding” of individual sails.
Flat, or working sails, are those sails used to propel a sailing vessel as close as possible into the wind, tacking, or at wider angles across the wind, reaching. Sails in this category include mainsails, jibs, Genoas, and a variety of other sails that usually have at least one edge attached to a mast, boom, or wire running from a spar to the vessel. In this realm of working sails are two categories of sailcloth that can be defined as woven and laminated cloth.
Woven Sailcloth & Cross-Cut Panel Layouts
Woven sailcloth is typically made with continuous filament polyester, such as Dacron, on looms that permit very dense constructions. This style of cloth is created by an over-then-under intersection of warp (fibers running the length of the roll) and weft (fibers running across the width of the roll) yarns that are tightly packed together. By varying the size, or denier, of these fibers in conjunction with the fiber count in either direction, the stretch properties of the cloth may be altered to better suit a particular sail design. For example, by combining a relatively large weft yarn with a small warp yarn, all the crimp displacement can be allocated to the warp yarn, in effect, the loom is bending the warps around the weft fibers that are being held straight. The result is a low stretch fabric in the weft direction with the warp direction easier to stretch as the added length of crimp gets pulled out of the fiber with loading.
With the exception of some experiments using tri-axial weaves which were symmetrical about the warp and the 45° axis, conventional woven sailcloth has been based upon a symmetric warp—weft fiber orientation of 0° for the warp and 90° for the weft. These fabrics are most commonly arranged in cross-cut panel configurations to align the stronger weft fibers up the leech of the sail in the general direction of the loading out of the head and clew corners of the sail. For the loads not following this path, the stability of the woven cloth generated by the very tight weave and resin finish helps reduce off thread-line stretch and promotes recovery from any cloth elongation that does occur. While some woven sail fabrics are designed to be used in radial constructions, the vast majority of sails made from woven Dacron are designed with cross-cut panel layouts. Because the woven cloth is typically symmetrical about the warp axis or machine direction, the fabric can be rolled either way in the panel, leech to luff or luff to leech, and still maintain the desired alignment of the weft fibers to the loads. However, the symmetry of the woven cloth can also be inefficient, since various fibers in the cloth may not be aligned to a load.
An exemplary cross-cut sail 100 is illustrated in FIG. 1(a). The fabric layout in an exemplary panel 102 of the cross-cut sail is illustrated in FIG. 1(b). Cross-cut sail constructions can vary in some details but the general panel layout has the seams of the cloth running from the leech of the sail to the luff at an angle roughly perpendicular to a straight line from the clew to the head of the sail. In so doing, the weft threads 106, which are usually lower in stretch than the warp fibers 104 in a cross-cut panel, are aligned with the strongest loads in the sail. Cross-cut panels 102 are generally rectangular in shape with one edge cut at an angle where it affixes to the mast. The cross-cut panels 102 are joined along their long edges which are slightly curved by the sailmaker to provide the necessary 3-dimensional shaping in the finished sail to generate lift.
Because the panels are generally rectangular, they can be cut from rolls of woven fabric with relatively little wasted material. The full width of the cloth can be utilized and with careful nesting of the panels to align the short edges together, it is not uncommon to achieve fabric utilization of 90-95%, meaning, for example, the sailmaker will need to order 55 yards of fabric for a sail that will ultimately use 50 yards of material.
Laminated Sailcloth and Tri-Radial Layouts
In the early 1980's, higher modulus fibers like Kevlar were introduced to sailcloth manufacturers. With considerably more strength and lower stretch than polyester Dacron fibers, Kevlar was first trialed in high performance sails for the America's Cup. Prone to flex fatigue and having no shrinkage even with high temperatures, this new fiber did not lend itself to the traditional tightly woven constructions used with Dacron and further tightened up with shrinkage through heat-setting.
Rather, this new fiber was woven into more loosely designed constructions that were stabilized with the addition of a Mylar film. By laminating a sheet of 2 or 3 mil film to these Kevlar taffetas, the cloth manufacturers imparted enough stability to the weaves to resist the off-thread line loading and allow the sails to hold their designed shapes. Early trials were plagued with failures in lamination and actual cloth breakage but improvements in lamination techniques and a better understanding of fiber content to strength requirements has all but eliminated those problems.
Concurrent to the advent of laminated cloth was the development of the tri-radial panel construction, first used in sails for the America's Cup. An exemplary tri-radial sail 201 is shown in FIG. 2(a), while the fabric layout of an exemplary panel 202 is shown in FIG. 2(b). This construction technique allows the sailmaker to align the warp thread line 204 of the cloth in the general direction of the loads emanating out of the sail corners. Hand in hand with this trend, cloth suppliers started to make warp oriented laminates where the majority of the fabric's strength was in the warp thread 204 direction and not in the direction of the weft threads 206. Because laminators can keep a high rate of tension on the warp fibers during manufacture, the stretch resistance of the warp could be optimized to limit fabric elongation to levels not possible in woven cloth. This new construction technique combined with the use of high modulus fibers in warp oriented laminates resulted in a new level of performance in terms of strength to weight and shape retention.
However, tri-radial design and panel configurations increases the labor required to build a sail and the amount of cloth needed to make all the panels. Because radial panels 202, or gores, are generally triangular in shape, they are not as efficiently nested into the sail as is a cross-cut panel. Furthermore, to maintain thread lines in the gores, even with tight panel nesting, a higher percentage of sail fabric is wasted when compared to the cloth utilization with cross-cut panels. Average cloth utilization rates for common tri-radial panel designs are in the 80% range meaning a sailmaker will need to use almost 63 yards of fabric for the same sail that used 55 yards in the cross-cut design example. Compounding the added expense of poor cloth utilization in the tri-radial design is the increased labor required for the assembly of the many more panels than needed for the same sail build as a cross-cut design.
For grand prix racing, where price is not a paramount concern to the sail maker or the sailor and where conventional roll good sailcloth is no longer widely used, some advances have been very successful. These include Peter G. Conrad's “airframe” patent, U.S. Pat. No. 4,593,639, and his “Genesis” patent, U.S. Pat. No. 4,708,080, which constructs a sail “without sailcloth”; and Jeanne-Pierre Baudet's molded sail construction methods, U.S. Pat. No. 5,097,784, U.S. Pat. No. 6,112,689 (used in North Sails' highly regarded 3DL® sails). Other patents, such as Fred Aivars Keir's woven laminate patent, U.S. Pat. No. 6,311,633, may also advance grand prix sailmaking. However, the cost of fabrication and technology required for these advances is beyond the resources of most sail makers and similarly beyond the budgets of most amateur sailors, even serious recreational racing and offshore cruising sailors.
It therefore is desirable for a sailcloth to be both affordable and less susceptible to load force stretch, creep elongation, and airfoil shape deformation.